Process for producing nonoriented silicon steel sheets



1965 KENJl TAKAHASHI 3,203,839

PROCESS FOR PRODUCING NONORIENTED SILICON STEEL SHEETS Filed Feb. 18, 1963 3 Sheets-Sheet 1 FIG.|

INVENTOR BY Kenji Takahashi PROCESS FOR PRODUCING NONORIENTED SILICON STEEL SHEETS Filed Feb. 18. 1963 Aug. 31, 1965 KENJI TAKAHASHI 3 Sheets-Sheet 2 FIG. 3

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PROCESS FOR PRODUCING NONORIENTED SILICON STEEL SHEETS Filed Feb. 18, 1963 3 Sheets-Sheet 3 FIG. 4

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Magnetizing force H in Oe INVENTOR Kenji Takahashi Y M 20 4 4d pamuew United States Patent 9 3,203,839 PROCESS FOR PRODUCING NONORHENTED SILICON STEEL SHEETS Kenji Takahashi, Tsukita, Yawata, Japan, assignor to Yawata Iron 8: Steel (10., Ltd, Tokyo, Japan, a corporation of Japan Fiied Feb. 18, 1963, Ser. No. 259,190 Claims priority, application Japan, Feb. 23, 1962, 37 7,027 1 Claim. (Cl. 148-113) This invention relates to a process for producing nonoriented silicon steel sheets having properties most adapted for an iron core material specifically for rotary electric machines.

Generally silicon steel sheets are largely classified by the producing processes into hot-rolled silicon steel sheets and cold-rolled silicon steel sheets. The main rolling for the reduction of the thickness of the hot-rolled silicon steel sheet is carried out at a high temperature above 700 C. and most of the crystal grains of the produced steel sheet are arranged in disorder and are so-called nonoriented. On the other hand, unless the cold-rolled silicon steel sheet so treated to reduce the thickness is specially worked, e.g., the so-called strain treatment (U.S. Pat. 2,236,519), it will become a so-called anisotropic steel sheet in which the characteristics parallel to the rolling direction and those transverse to the rolling direction are different from each other.

As compared with the steel sheet produced by the hotrolling process, the steel sheet produced by the coldrolling process has advantages in that it can be made longer and is therefore not only economically very advantageous in the production of the steel sheet but also is uniform and the surface of such sheet is smooth and beautiful and the space factor may be far elevated in assembling an iron core.

Generally, in the oriented silicon steel sheet, most of the crystal grains forming the steel sheet are so arranged by directing the direction of easy magnetization 100 direction) in the rolling direction or in both the rolling direction and the transverse direction. It is therefore very effective for an iron core for electric machines in which such steel sheet can be used as magnetized in both the rolling direction and the direction at right angles thereto. However, in the oriented silicon steel sheet, the magnetic characteristics in any other direction is far inferior to that in the rolling direction or the direction at right angles thereto. Thus it has no uniform characteristic in every direction. Therefore, it can not help being disadvantageous as an iron core for rotary machines and any other electric machines forming complicated magnetic paths. Thus, it is not only required from the characteristics of electric machines but also convenient in designing machines, to produce an iron core for rotary electric machines which has as little difference as possible when magnetized in the rolling direction and when magnetized in another direction or is so-called nonoriented.

Processes for producing nonoriented silicon steel sheets by cold rolling have been studied of late. One of the known processes is a process wherein, when less than about 3% silicon is added, the growth of oriented crystals is inhibited by utilizing the 11- lattice transformation of the silicon steel in finish-annealing. Said process is intended specifically for once strong cold rolling having no intermediate annealing. Therefore, as the chance of de- 'ice carburizing is relatively small, there is a defect that, unless a steel of good quality is used, no favorable results are generally obtained in general magnetic characteristics.

Further, another process shows that, by introducing the skin pass rolling process of a little higher strength (the so-called strain treatment) before the final step, the ani sotrophy resulting from the previous cold rolling and the intermediate annealing may be broken and thereby a nonoriented steel sheet can be produced. However, in this process, it often happens, that, in some cases, the anisotrophy will remain and the nonorientation will be insufiicient.

Moreover, anisotrophy will occur because of delicate skin pass rolling.

An object of the present invention is to provide a process wherein nonoriented silicon steel sheets high in magnetic characteristics are obtained as uniform stable products by a simple process.

Another object of the present invention is to provide a process wherein nonoriented silicon steel sheets high in magnetic characteristics are obtained by hot-rolling an Fe-Si-Al steel having a specific composition, in a known manner, and then two stage cold-rolling and heat-treating the steel.

A further object of the present invention is to provide a process wherein nonoriented silicon steel sheets high in magnetic characteristics are obtained by carrying out the above mentioned cold-rolling step one to more than three times.

FIGURES 1 and 2 show the pole figures of planes prepared with X-rays on the crystal grains of nonoriented magnetic steel sheets treated by the process of the present invention.

FIGURE 3 shows magnetic torque curves of nonoriented silicon steel sheets produced by the process of the present invention, and FIGURE 4 shows magnetization curves of nonoriented silicon steel sheets produced by the process of the present invention as compared respectively with the characteristics of a commercial nonoriented silicon steel sheet.

The present invention shall be described in greater detail below.

The process of the present invention comprises the step of hot-rolling into a steel sheet, according to a process known per se, an ingot composed of 15-35% Si and 0.5-1.5% Al, the rest being Fe and unavoidable impurities and the sum of Si and Al content being in the range of 25-45%, the step of cold-rolling said steel sheet at a reduction rate of 50-80% and intermediately annealing it at a temperature of from 750-950 C. in a reductive or neutral atmosphere for 3 minutes to 1 hour and the step of cold'rolling said steel sheet at a reduction rate of 50-80% and finally annealing it at LOGO-1,100 C. for 5-40 hours.

The contents of silicon and aluminum in the present invention shall be explained. As a result of experiments, the sum of the contents of silicon and aluminum is made substantially 25-45%. If this sum is less than 2.5%, the specific resistance will be reduced and, as a result, the core loss will increase. If it is more than 4.5%, the cold-rolling will be difiicult and, therefore, not only the production will be more difiicult but also the magnetic induction will be reduced. Further, it has become evident that, in such case, in order that the magnetic characteristics may be good and satisfy the conditions of nonorientation and other conditions, the silicon content should be kept higher than, or at least equal to, the aluminum content.

In order to prepare an ingot having such a composition of silicon and aluminum as above mentioned, pure iron is melted in an Ar atmosphere in a high frequency furnace in order to have detrimental impurities reduced, and is then kept in a vacuum. The composition is adjusted so that the silicon content is from 1.5 to 3.5%, the aluminum content is from 0.5 to 2.0%, the sum of both added elements is within the range of 2.5 to 4.5% and the silicon content is higher than the aluminum content. The so-produced steel is then tapped and is made an ingot. The composition ranges of the ingots used in the experiments were as in Table 1.

T able 1 .-Cmposili0ns 0f ingots The ingot having such composition is soaked at 1,200 C. in a soaking furnace in a manner known, per se, and is forged or is merely divided into blooms. Such bloom is heated at l,1501,250 C. (which heating temperature is not specifically defined) for minutes and is hot-rolled at a rolling finishing temperature of 850- 900 C. to be about 2.5 mm. thick according to the object product. The hot-rolled steel sheet is pickled with a 15% H 80 solution at about 80 C. to remove any oxidized film deposited on the surface during the hotrolling. Thus the thickness of the steel sheet will be reducedby about 0.2 mm. This steel sheet is cold-rolled twice. In such case, the proper reduction rate by the cold-rolling, each time, is 5080% but may be diiferent depending on the thickness of the final product. In case room temperature rolling is difiicult due to brittleness, the steel sheet may be heated to about 200 C. and then rolled. Thus the thickness of the sheet will become about 1 to 0.2 mm. The above mentioned intermediate annealing is then carried out at a temperature of 750 to 950 C. for a short time (less than about minutes) in an NH crackered gas having a dew point of about +30 C. In such case, the atmosphere need not always be the above mentioned gas but may be a simple inert atmosphere. However, nitrogen gas is likely to form aluminum nitride which is very detrimental to the magnetic property and is not recommendable. It is effective to attempt decarburization by elevating the dew point of the atmosphere. In such case, the heat-treating temperature and time will have such great influence on the characteristics expected in the product that they must be selected in accordance with the required magnetic characteristics. Especially, when a nonoriented steel sheet is produced, it the above mentioned intermediate annealing temperature is low, the time of the heat-treatment will have to be for a comparatively long time (about 60 minutes) but, if said temperature is as high as 900 or 950 C., the heat-treatment will have to be for a short time of less than about 3 minutes. The finish annealing is carried out in an atmosphere having a low dew point (below C.) which may be either inert or reductive or in a vacuum below 10- mm. Hg. However, a nitrogen atmosphere has such a bad influence on the magnetic property that it should not be used. An annealing temperature of about 1,000-1,100 C. will give favorable results. An annealing temperature above 1100 C. may give orientation and is not desirable. The annealing time is not specifically limited but should be comparatively long (about 5 to 40 hours). The cooling rate should be such so as to give no undesirable internal strain in the steel sheet or less than about 50 C./hr.

The process of twice cold-rolling has been described supra. However, in the process of the present invention, the intermediate annealing may be omitted and the above mentioned cold-rolling may be done only once with a reduction rate of 50-85% Further, the object product may be obtained by carrying out the cold-rolling more than three times.

The magnetic characteristics and the structure of the product obtained by the present invention shall now be explained. FIGURES 1 and 2 represent the examples of the (110') pole figures of crystals which form steel sheet, analyzing samples No. 4 in Table 3 and No. 10 in Table 4 with X-rays (using MoKot line) and the pole of the rolled plane being the north pole, R.D. being the rolling direction and TD. being the direction transverse to the rolling direction.

It is seen in said pole figures that the concentration of the (110) pole is weak and is not in a fixed place. It can be substantially said that some of the (111) plane of the crystals are seen to be parallel to the rolling plane but do not take any specific direction and weak structure of the deviated [001] type. Some other examples show weak structure of the (111)[1l2] type.

It is considered that the structure of the final product has such (111) plane structure and a small amount of weak structure of the deviated (100) [001] type are present and that the others are arranged in disorder. FIGURE 3 show magnetic torque curves of sample Nos. 4(IV) and 10(X) and a commercial product (P). The maximum magnetic torque is about 3x10 dynecm./cm. at most and is very low, showing the facts above mentioned.

Table 2 shows magnetic characteristics of a commercial cold-rolled nonoriented silicon steel sheet.

As evident from said table, there are considerable differences between the values of magnetic characteristics in the rolling direction L and those in the direction C transverse to the rolling. In the core loss of W10/50 which is an important magnetic amount as of an iron core, as compared with the value of 0.81 W/kg. in the direction L, the value in the direction C is 1.27 W/kg. and is higher by 57%. The maximum magnetic permeability ,u is 15,900 in the direction L and 5,900 in the direction C and is therefore lower in the direction C by as much as 63%. There are considerable diiference also in the other characteristics. Thus, the commercial steel sheet has considerable anisotropic magnetic characteristics and, therefore, can not be said to be nonoriented. On the other hand, as shown in Table 3, the product produced by the process of the present invention shows superior nonorientation character than the commerical product and its magnetic characteristics are also favorable. Table 3 and Table 4 show characteristics of examples of different contents of silicon and aluminum in the composition range of Example 1 and Example 2 in Table 1 respectively.

As evident from this, it is seen that the core loss value is about 0.80 W/kg. in the direction L and about 0.90 W/kg. in the direction C and is only about 10% larger in the direction C. It is evident that nonorientation is improved much more than in the commercial product in Table 2. Further, the difference between the maximum magnetic permeability in one direction and that in the other is also less than about 30%. Thus, the magnetic anisotropy is far smaller. In FIGURE 3, P is a magnetic torque curve of a commercial nonoriented silicon steel sheet and IV and X are magnetic torque curves of steel sheets produced by the process of the present invention (IV represents a sample No. 4 in Table 3 and X represents a sample No. 10 in Table 4). The maximum magnetic torque is 5x10 dyne-cm./cm. in the fromer but is less than 3x10 dyne-cm./cm. and evidently lower in the latter. It is thus evident that the latter is less oriented and has characteristics close to nonorientation. A sample No. 7 and those following it in Table 3 show various characteristics in the case of different contents of silicon and aluminum in the composition range in Example 2 in Table 1 in which the carbon content is reduced.

As evident from this, it is found not only that the maximum magnetic torque is still low (about 3x10 dynecm./cm. and the steel sheet is nonoriented but also that the value of the core loss W10/ 15 is low (0.6 W/kg.) and the characteristics of the magnetization are excellent in direction C as well as in direction L. FIGURE 4 shows the characteristics in the direction L (QL) and the characteristics in the direction C (QC) of the example of a sample No. 5 (which shall be represented as sample Q) in the example in Table 3 in the present invention as compared with the characteristics in the direction L (PL) and the characteristics in the direction C (PC) of the commercial nonoriented silicon steel sheet P. It is shown that the low magnetic induction is improved very much in the direction C though high magnetic induction at higher magnetic field is a little lowered in the direction L, but that the difference between them in the direction L and C is very small.

In Tables 2, 3 and 4, H and H, denote the coersive force at max. magnetic induction 10,000 and 15,000 guasses respectively, B, and B,- the residual magnetic induction at max. Magnetic induction 10,000 and 15,000 gausses respectively, W10/50 and W15/50 the core loss in watts per kilogram at max. induction 10,000 and 15,000 guasses and at 50 c.p.s. and B B and B magnetic induction in gausses at magnetizing field 10, 25 and 50 oersted respectively.

Table 2.Magnetic characteristics of commercial cola-v rolled nonoriented silicon steel sheet Maximum Values of magnetic characteristics magnetic torque Coercive Residual Maximum force in magnetic Core loss in W/kg. magnetic Magnetic induct-ion e induction permein gausses dyne-cmJ in gausses ability cm. M

Ha H. BM 13, W15/50 W15/50 m Bm B25 B50 In the rolling direction L 0. 42 0.56 8, 600 12, 000 0. 81 2.07 15,900 14,900 15, 900 16, 700 4. 4X10 In the direction 0 transverse to rolling direction 0. 65 0.80 6, 100 7, 400 1. 27 2.92 5, 900 13, 700 14, 900 15,600 4. 4X10 I Mean values in the directions L and C- 0.54 0. 68 7, 400 9,700 1. 04 2. 50 10,900 14, 300 15, 400 16, 200 4. 4X10 (C-L/L) percent +55 +45 29 -38 +57 +41 -63 8 6 7 4. 4x10 Table 3.-Magnetic characteristics of products produced by the process of the present invention Finish, Values of alternating current magnetic characteristics (50 cycles) Intermediannealing ate annealconditions Maxi- Samples, ing conditemperature Residual mum Sample percent tions, temin C X Rolling Coercive magnetic Core loss in Maximum Magnetic induction magnetic N o. perature time in direction force in De induction W/kg. magnetic in gausses torque,

C.X time hours, (1001- m gausses permeadynein minutes ing 117158.130 v bility, m cm./crn.a

r. at Al i Hm H, 13. B. W10/50 W15/50 B10 B B50 0. 44 0. 63 8, 500 12,000 0. 81 2. 61 13, 300 13,800 14, 600 15,400 1 3. 2 0. 6 3.8 75OX3 1000Xl0. 25 0. 49 0. 71 7, 500 10, 900 0.90 3. 44 10, 000 13,400 14,100 14,900 1 7 0. 45 0. 62 8,100 12,000 0. S2 2. 52 12,200 14,000 14, 800 15, 500 2 3. 2 0. 6 3.8 75BX30 1000X10. 25 0. 48 0. 69 7, 900 10, 900 0. 88 3. 14 10, 700 13, 500 14,300 15, 000 1 9 1 L10 Mean of L and C (C-L/L) X100.

Table 4.Magnetic characteristics of products produced by the process of the present invention Samples, percent Values of alternating current magnetic characteristics Intermediate Finish, Maximum annealing annealing magnetic Samconditions, conditions, Residual torque, plo temperature temperature Coercive magnetic Core loss in Maximum Magnetic induction in dyne- No. Al in 0.x in 0.x force in O. induction W/kg. magnetic gausses cnr/cm.

Si Al time in time in hours, in gausses permea- Si minutes cooling rate bility, (X10 in C./l1r. m

Ho 11, 13, B. W10/50 W15/50 Bro B2 B 7 2.65 0. 47 3. 12 SOOXO. 1, 000X10, 50 0. 38 0. 54 8, 000 12, 500 0. 65 2. 21 15,700 14,400 15, 200 15, 900 2. 2 8 2. 26 0. 53 2. 79 800 1 1, 000x10, 50 O. 39 D. 55 7, 700 11, 600 0. 69 1. 99 14, 000 14, 400 15, 100 15, 800 1. 6 9 3. 18 0. 78 3. 96 900X3 1, 000x10, 50 0. 3G 0. 51 8, 600 12, 500 0. 64 1. 95 18, 300 14, 400 15, 100 15, 800 3. 8 2. 98 0. 78 3. 76 None 1, 000x10, 0. 40 0. 56 8,600 12, 700 0. 68 2. 08 17, 600 14,400 15, 200 16, 000 1. 9 11 2. 98 O. 81 3. 79 900x10 1, 000x10, 50 0.36 0. 50 7, 600 10, 500 0. 60 1. 81 14, 700 14, 700 15, 600 16, 300 3. 0 12 1. 92 0.98 2. 90 None 1, 000x10, 25 0. 41 0. 58 8,700 12, 800 0. 73 2. 17 17, 200 14, 500 15, 200 15,800 2. 3 13 1.35 1. 77 3. 12 950X1 1, 000x10, 50 0.35 0. 50 7. 500 10, 600 0. 62 1. 93 15, 600 14, 200 15, 000 15, 800 3. 1

Example-The composition of a vacuum-smelted 100 kg.-ingot was as follows (weight percent):

3.0% Si, 0.5% A1, less than 0.03% C, about 0.2% Mn, less than 0.005% P, less than 0.01% S and less than 0.01% Cu.

The above mentioned ingot was forged into a slab 7 mm. thick and 220 mm. wide. The sla'b was hot-rolled to the gauge of 2.3 mm. It was cold-rolled for the first stage at reduction rate of 60%, was intermediately annealed and was further cold-rolled for the second stage at a reduction rate of 70% to be a final gauge 0.3 mm. thick.

Intermediate annealing conditions:

At 750 C. for less than minutes At 800 C. for less than 10 minutes At 850 C. I At 900 C. j

Finish-annealing conditions:

At 1,000-1,l00 C. for 10-40 hours Cooling rate: 50 C./hr. Atmosphere: Argon (dew point- C.)

for less than 3 minutes 1 Resnlts.The main magnetic characteristics in case the maximum magnetic torque M was less than 3x10 dynecm./cn1. were in the following ranges in all the above mentioned conditions:

What I claim is:

A process for producing a non-oriented silicon steel sheet in which the crystal grains are in a state of random alignment and, when magnetized, has equal degree of magnetization in all directions, which comprises ('1) hotrolling an ingot to produce a steel sheet, said ingot consisting essentially of 1.5 to 3.5 wt. percent Si, 1.09 to 1.34 wt. percent Al, and Fe, the sum of Si and Al being from 2.5 to 4.5 wt. percent, and the amount of Si in said ingot being greater than the amount of Al, (2) coldrolling the steel sheet to effect reduction of 50 to in thickness, (3) intermediately annealing the cold-rolled sheet at a temperature of from 750 to 950 C. in a nonoxidizing atmosphere for 30 to 3 minutes, (4) cold-rolling the annealed sheet to effect a reduction of 50 to 80% in thickness, (5) and finally annealing the cold-rolled sheet at a temperature of 1000 to 1100 C. in a non-oxidizing atmosphere for 5 to 40,hours.

References Cited by the Examiner UNITED STATES PATENTS 2,307,391 1/43 Cole et a1 148110 2,378,321 6/45 Pakka'la 148-111 2,875,114 2/59 Albert 148120 2,965,526 12/60 Wiener 148111 FOREIGN PATENTS 1,009,214 3/54 Germany.

870,220 6/61 Great Britain.

DAVID L. RECK, Primary Examiner. 

